This invention relates to an angle measuring instrument and particularly to an angle measuring instrument which has two measuring arms pivoted to a protractor plate and an additional arm pivoted to the two measuring arms, more particularly to one having a graduated plate member of specific configuration with its sides being used as measuring sides.
The angle measuring instrument of the type described above is disclosed in U.S. Pat. Nos. 2,090,835 to Gardhman, 2,607,121 to Wiedebusch and 2,735,184 to Chamberlain and German Pat. No. 1073756 to Nurchis. However, the constructions of the instruments disclosed in these patents are quite different from the present invention except for Nurchis' instrument which is deemed more relevant. The Nurchis' instrument includes a substantially semi-circular protractor plate and three measuring arms forming a parallelogram with the plate. Although the Nurchis' construction is simple and is suitable for measuring angles of a variety of shapes, there is a drawback in that the Nurchis' instrument can not measure an angle of an object which has therein an obstruction that prevents the two pivoted measuring arms of Nurchis from extending into the angle to be measured. An example of such an angle is that formed at a truncated corner, or that formed by two planes or two edges which incline with one another but do not intersect one another, and a third plane or edge intersects the two planes or edges near where the corner of the planes or edges is truncated.
Wiedebusch's instrument can be applied to measure the above described angles. However, Wiedebusch's construction is more complicated than Nurchis, including a protractor plate and four pivoted measuring arms which form a linkage system. Wiedebusch's construction requires complicated processing steps to obtain a precision measuring linkage system.